1. Field of the Invention
This invention relates to iron-zinc electrogalvanized metal sheet and strip, and particularly to improved methods and electrolytes for providing electrogalvanized steel sheets and strip having enhanced surface smoothness essentially free of "mounds" caused by metal oxides occluded in the electrodeposited metal coating.
2. Description of Related Art
Manufacture of lustrous iron-zinc electrodeposited coatings on metal substrates is described in Salt, U.S. Pat. No. 2,778,787. In that patent, such deposits are obtained from an electrolyte bath containing chlorides of iron, zinc, ammonia and potassium and, for example, 0.5 g/l of citric acid which furnishes ferric ion with a chelate group, thereby preventing precipitation of ferric hydroxide from the bath.
In an article entitled "Development of Zn-Fe Alloy Electroplating With Soluble Anode in Chloride Bath, " 4th AES Continuous Strip Plating Symposium, Chicago, Ill., May 13, 1984, Irie et al. describe a similar process using soluble anodes and with addition of citric acid (amount not specified) to prevent precipitation of ferric hydroxide from an electrolyte bath containing chlorides of iron, zinc and ammonia.
Japanese published application No. 59/211594 discloses the production of Zn-Fe electroplated steel sheets wherein the deposited coating contains 7-35 weight percent Fe and is chemically single-phase. Examples of electrolyte baths contain chlorides of iron, zinc, ammonia and either ammonium citrate, 5 g/l, or citric acid, 2 g/l, together with sodium acetate, 10 g/l, and wherein plating was performed at 50.degree. C. (122.degree. F.) and a pH of 3.
U.S. Pat. No. 4,540,472 discloses the electrodeposition of Zn-Fe alloy coatings from an electrolyte bath containing chlorides of zinc, iron and potassium, together with an amount of sulfate ion, an adduct such as a polyethylene glycol, and a chelating agent such as citric acid in an amount from 0.5 g/l to 5 g/l.